Various technology related to forming thin films on substrate materials has been available for many years. However, recent product requirements have imposed demands which existing technology could not adequately satisfy. Ever increasing demand for high quality films, substantially free of pinhole-type defects, has occurred in the field of photovoltaics and of electrical display devices.
A major use of thin films in the prior art was coated glass for use in buildings. The coatings were applied to obtain a variety of colors for architectural purposes and for screening incident radiation. Such coatings required a degree of optical clarity and freedom from interference rings, but did not require a high degree of perfection with respect to pinhole defects. Electrically conductive coatings were applied to transparent substrates for some applications, e.g., cockpit windows in airplanes to provide for passing an electrical current for heating the window, where additives could be used to obtain the desired film characteristics, such as light transmission and electrical conductivity.
Recent developments in photovoltaics and in electrical displays have produced requirements for quantities of high quality films heretofore produced only for low volume speciality applications. These requirements are for thin films up to 4 microns in thickness formed on vitreous substrates, such as glass. Extremely thin films are required for optical purposes as well as for conservation of scarce materials.
In the prior art, such thin films have been formed by various techniques, including vacuum evaporation and deposition, vapor deposition, high frequency sputtering, and liquid sprays onto a heated substrate. The present application concerns only the application of liquid sprays onto a heated substrate, and more particularly, a substrate which is heated throughout the spray process.
The prior art is replete with attempts to obtain high quality film deposition using spray techniques. Spraying offers the potential of providing a high volume, low cost process which may be readily adapted to quantity production. However, spray application can produce a number of difficulties with respect to forming high quality films.
High quality film formation generally occurs when the sprayed materials maintain relatively similar time-temperature histories. Sprayed materials which reach the surface or a position adjacent the surface, which are relatively longer in transit, or which are deposited adjacent surface portions with relatively different temperatures form films having quite different characteristics than desired. Thus, it would be desirable to provide film-forming material adjacent the surface with controlled time-temperature histories and to confine deposition of that material to selected portions of the heated substrate.
One such attempt in the prior art is described in U.S. patent application Ser. No. 886,891 wherein the spray nozzle is tilted with respect to the vertical to confine the sprayed solution to a selected portion of the substrate and wherein various baffles are provided to shield the substrate in regions where the substrate temperature was not sufficiently controlled or where the accumulated time-temperature history of the particles produced unacceptable films. Thus, only a portion of the spray was selected for deposition on the surface.
Yet another technique is to provide exhaust ducts adjacent the spray nozzles for removing reaction products above the surface. Such device, by way of example, is depicted in U.S. Pat. No. 3,887,349 for forming metallic oxide coatings on a glass surface. Another approach disclosed in U.S. Pat. No. 4,125,391 is to project the spray at a relatively low angle of incidence with respect to the surface in the direction of substrate movement. Exhaust ducts are provided downstream from the spray to collect reaction products, but without affecting the spray prior to the spray impinging on the surface.
The prior art devices to attempt to remove reaction products from above the glass surface. There is, however, generally no control of the time-temperature history of the droplets other than gross removal afforded by the exhaust system. Thus, metallic oxide films are produced which are satisfactory for window glass, but are not satisfactory for high quality transparent electrically conductive films. Yet another prior art technique discussed in U.S. Pat. No. 3,880,633, is to produce a spray composed of uniformly sized drops. However, such a system is very difficult to obtain with an air atomized spray system. Even with uniform droplet sizes, a time-temperature history control is still desirable to maintain a product of uniform quality.
The disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus for forming thin films by a spray deposition process are provided.